Secondary batteries can be used under repeated charge and discharge. Therefore, secondary batteries are useful for reducing waste products, and widely used as power sources for portable devices that cannot use an alternating-current (AC) power source, and power sources for backup in case of disconnection or stoppage of AC power sources. In recent years, an expansion in the range of applications has been considered, such as an in-vehicle application, backup for solar cells and the like, and power leveling application, and there are accordingly increasing demands for improvements in performance such as capacity, temperature characteristics, and safety.
Among secondary batteries, the term nonaqueous electrolyte secondary batteries refers to secondary batteries that are charged and discharged by the movement of lithium ions between positive and negative electrodes. In addition, nonaqueous electrolyte secondary batteries include electrolytic solutions including an organic solvent, and thus have a feature of being able to achieve higher voltages than nickel-cadmium secondary batteries and nickel-metal-hydride secondary batteries that use aqueous solutions. Currently, lithium-containing cobalt composite oxides and lithium-containing nickel composite oxides are used as positive electrode active materials for nonaqueous electrolyte secondary batteries in practical use. Carbon-based materials and titanium-containing oxides are used as negative electrode active materials. In addition, lithium salts such as LiPF6 or LiBF4 dissolved in organic solvents such as cyclic carbonate and chain carbonate are used as electrolytic solutions. The positive electrode active materials have, with respect to the lithium metal potential, an average operation potential on the order of 3.4 to 3.8 V (vs. Li/Li+) and a maximum possible potential of 4.1 to 4.3 V (vs. Li/Li+) in the case of charge. On the other hand, the carbon-based materials as negative electrode active materials achieve a potential on the order of 0.05 to 0.5 V (vs. Li/Li+) with respect to the lithium metal potential, whereas the most typical lithium titanate (Li4Ti5O12) achieves is 1.55 V (vs. Li/Li+) among the titanium-containing composite oxides. The combinations of the positive and negative electrode active materials achieve battery voltages of 2.2 V to 3.8 V and a maximum charge voltage of 2.7 V to 4.3 V.
Secondary batteries that include titanium-containing oxides for negative electrodes can increase the charge-and-discharge cycle life, and such batteries have been put into practical use. However, while conventional lifespans of 2 to 3 years have been requested for mobile device applications, that for in-vehicle applications and stationary applications related to power generation is requested to be 10 years or more. Correspondingly, a further improvement in such secondary batteries is required. The most representative cycle performance of life performance is expressed as a decrease in battery capacity in the case of repeating charge and discharge, and the 2 to 3 years for mobile device applications corresponds to hundreds of times, whereas the 10 years or longer for in-vehicle and stationary applications corresponds to several or ten thousand times or more.
The main cause of cycle deterioration has not been clarified for batteries that include titanium-containing oxides as the negative electrode active material.